1. Field of the Invention
The present invention relates to a piezoelectric oscillator for use in gyroscopes, and more particularly, relates to a piezoelectric oscillator in which the number of electrodes is reduced, and relates to a signal detection apparatus using the same.
2. Description of the Related Art
FIG. 15A is a perspective view showing one of the surfaces of a conventional three-pronged tuning-fork-type piezoelectric oscillator. FIG. 15B is a perspective view showing the other surface thereof. FIG. 16 is a front view of a piezoelectric oscillator when the piezoelectric oscillator of FIG. 15A is viewed from the direction of arrow 16.
The whole of an oscillator 1 is formed from a plate-shaped piezoelectric material, such as a piezoelectric ceramic or a quartz crystal; a tip portion thereof is separated by slots 1A and 1A, and three vibration legs 1a, 1b, and 1c are integrally formed therewith. The dielectric polarization direction of the piezoelectric material in each of the vibration legs 1a, 1b, and 1c is as indicated by the arrows in FIG. 16. The dielectric polarization direction is the same for the vibration legs 1b and 1c on both right and left sides. In the center vibration leg 1a, the dielectric polarization direction is laterally and vertically symmetrical with respect to the vibration legs 1b and 1c on the right and left.
The lower surface of each of the vibration legs 1a, 1b, and 1c is formed with a pair of driving electrodes 2 and 2, using a conductive material. The driving electrodes 2, 2, . . . , extend up to the base end portion 1B of the oscillator 1, as shown in FIG. 15B. These driving electrodes 2, 2, . . . , are connected to an AC driving power supply 3 by wiring (not shown), so that a driving voltage of the same electrical potential is supplied to the driving electrodes 2, 2, . . . , from the AC driving power supply 3.
On the upper surface of the oscillator 1, a pair of ground electrodes 4 and 4 are formed in each of the vibration legs 1b and 1c on the right and left, and one ground electrode 4 is formed in the center vibration leg 1a. The driving electrodes 4, 4, . . . , extend to the base end portion 1B of the oscillator 1. On the surface of the oscillator 1 shown in FIG. 15A, a concentration pattern 4a is formed in the base end portion 1B, and all the ground electrodes 4 are connected to the concentration pattern 4a. Each of the ground electrodes 4 is at the ground potential by a wiring path (not shown).
The driving electrodes 2, 2, . . . , and the ground electrodes 4, 4, . . . , supply a driving voltage to the piezoelectric material which is a driving means. Based on the dielectric polarization structure shown in FIG. 16, the vibration legs 1b and 1c on the right and left are driven to vibrate at the same phase in the X direction, and the vibration leg 1a in the center is driven to vibrate 180xc2x0 out of phase opposite to that of the vibration legs 1b and 1c on right and left similarly in the X direction. That is, at a particular point in time, the amplitude of the vibration legs 1b and 1c on the right and left in the X direction and the amplitude of the vibration leg 1a in the X direction are opposite.
The surface of the center vibration leg 1a is formed with a pair of detection electrodes 5a and 5b. The detection electrodes 5a and 5b extend more toward the front than the base end portion 1B in the back portion of the oscillator 1, and the detection electrodes 5a and 5b are formed integrally with land portions 5a1 and 5b1, respectively.
In a state in which the vibration legs 1a, 1b, and 1c are driven in the X direction, when the oscillator 1 is placed within a rotating system having an angular velocity xcfx89 about the Z axis, each of the vibration legs 1a, 1b, and 1c has a vibration component in the Y direction due to a Coriolis force. In the vibration legs 1b and 1c on both sides and the vibration leg 1a in the center, since the phases of vibrations by a driving voltage are opposite, the phases of vibrations by a Coriolis force are opposite between the vibration legs 1b and 1c and the vibration leg 1a. That is, at a particular point in time, the directions of the amplitude components, in the Y direction by the Coriolis forces, of the vibration legs 1b and 1c are the same, and the direction of the amplitude components in the Y direction, of the center vibration leg 1a is opposite to the direction of those of the vibration legs 1b and 1c. 
The detection electrodes 5a and 5b are formed on the same plane (the same vibration plane) of the center vibration leg 1a, and the piezoelectric material of the center vibration leg 1a functions as a Coriolis force detection means. The dielectric polarization directions of the piezoelectric materials in the portions where the detection electrodes 5a and 5b are formed are opposite to each other. Therefore, when each of the vibration legs 1a, 1b, and 1c is driven to vibrate in the X direction in accordance with a driving signal from the AC driving power supply 3, and when an angular velocity xcfx89 is given, the Coriolis output component by vibrations in the Y direction by a Coriolis force cause a phase difference xcfx86 to occur between a detection output C from the detection electrode 5a and a detection output D from the detection electrode 5b.
For these detection outputs C and D, a DC voltage corresponding to the phase difference xcfx86 is detected by a phase difference detection means (not shown), the angular velocity xcfx89 is determined from this DC voltage, and the angle is determined by numerical integration of this angular velocity xcfx89.
However, in the signal detection method in the above-described conventional gyroscope, there are problems such as those described below.
First, in the center vibration leg 1a, in addition to the detection electrode 5a and the detection electrode 5b, the ground electrode 4 is provided on the narrow surface thereof, and moreover, this ground electrode 4 must be formed parallel to the detection electrode 5a and the detection electrode 5b in an area from the tip of the vibration leg 1a up to the base end portion 1B.
However, it is difficult to evenly form the ground electrodes 4 at an equal spacing from both the detection electrodes 5a and 5b in the area from the tip of the vibration leg up to the base end portion 1B. Therefore, if the creeping distance Wa between the ground electrode 4 and the detection electrode 5a differs from the creeping distance Wb between the ground electrode 4 and the detection electrode 5b, there is a problem in that insulation breakdown is likely to occur between the ground electrode 4 and the detection electrode 5a or between the ground electrode 4 and the detection electrode 5b. 
The present invention has been achieved to solve the above-described conventional problems. An object of the present invention is to provide a piezoelectric oscillator in which the number of ground electrodes, which are required conventionally, can be reduced, simplifying a manufacturing process.
Another object of the present invention is to provide a piezoelectric oscillator in which electrodes are arranged appropriately in vibration legs so that a phase difference can be reliably detected, and to provide a signal detection apparatus using this piezoelectric oscillator.
To achieve the above-mentioned objects, according to the present invention, there is provided a piezoelectric oscillator for outputting an angular velocity proportional to a Coriolis force in a rotating system, the piezoelectric oscillator comprising: a vibration leg having a rectangular or square cross section; a pair of driving electrodes extending with a spacing therebetween in a direction in which the vibration leg is driven and in the direction of the length of the vibration leg in the plane extending in the driving direction; and a pair of output electrodes opposing the driving electrodes, extending in the direction of the length of the vibration leg, on a surface opposite to the surface on which the driving electrodes of the vibration leg are formed, wherein when driving power is supplied to the driving electrodes, the vibration leg is driven to vibrate by a piezoelectric effect, and an angular velocity component proportional to the Coriolis force is obtained from the pair of output electrodes by a piezoelectric effect.
In the present invention, a pair of driving electrodes may be formed on the surface of one of the vibration legs, a pair of output electrodes may be formed on the other surface thereof, and no ground electrode need be provided between the driving electrodes and the output electrodes. This makes it possible to avoid the conventional problem of having to make the creeping distance equal between the two output electrodes and the ground electrode. Therefore, the problem of insulation breakdown can be eliminated.
Also, since a ground electrode for the purpose of grounding, provided between a pair of output electrodes, can be omitted, it is possible to simplify the manufacturing process for forming electrodes.
In the foregoing, the oscillator is preferably provided with three-pronged vibration legs, the driving electrode and the output electrode are provided in all the vibration legs, and a driving signal is supplied between the driving electrode and the output electrode so that an angular velocity component proportional to a Coriolis force is detected from at least one set of output electrodes from among the output electrodes.
A description is given by using a piezoelectric oscillator shown in FIG. 2. The one set of output electrodes described above is, most preferably, a combination of a pair of output electrodes a and b provided in a center vibration leg (11v). In addition, for example, they may be a combination of output electrodes a1 and b1 of a vibration leg (11u) on the left side, a combination of output electrodes a2 and b2 of a vibration leg (11w) on the right side, a combination of the output electrode a1 of the vibration leg (11u) on the left side and the output electrode b2 of the vibration leg (11w) on the right side, or a combination of the output electrode b1 of the vibration leg (11u) on the left side and the output electrode b2 of the vibration leg (11w) on the right side. It is also possible to make the outputs of the output electrodes a1 and a2 and the outputs of the output electrode b1 and b2 one set of output electrodes.
Preferably, an output electrode other than the one set of output electrodes is fixed to an invariable potential.
For example, the invariable potential is a ground potential, and in addition, may also be a power-supply voltage, a midpoint potential (xc2xd of the power-supply voltage), etc., as long as it is an electrical potential which does not vary.
Also, when the axis which passes through the center of each vibration leg in the width direction and which extends in the direction of the length thereof is made a center axis of each vibration leg, the output electrode is preferably provided at a position which is farthest from the center axis in the plane of the vibration legs.
When the piezoelectric oscillator which is driven to vibrate is placed in a rotating system, a Coriolis force acts on each vibration leg so that the motion of each vibration leg becomes an elliptical motion. Therefore, since the position at which the displacement of the oscillator due to the Coriolis force becomes maximized is a position farthest from the center axis of each vibration leg, provision of an output electrode at this position allows a Coriolis force to be detected at higher sensitivity.
In addition, to achieve the above-mentioned objects, the present invention provides a signal detection apparatus comprising a piezoelectric oscillator comprising I/V (current/voltage) conversion means for converting a current output obtained from the output electrode into a voltage output, each of the output electrodes being grounded via an imaginary short-circuit in the I/V conversion means.
In the present invention, a pair of driving electrodes are formed on one of the surfaces of a vibration leg, and a pair of output electrodes are formed on the other surface thereof. An AC driving signal is supplied to the driving electrode, and each pair of electrodes disposed in the vibration legs on both sides within the other surface are connected to a ground potential or to an invariable potential, making it possible to vibrate the vibration legs on both sides in the same phase. Also, the electrode disposed in the center vibration leg is connected to the I/V conversion means comprising an operational amplifier, etc. Between an inversion terminal and a non-inversion terminal of the operational amplifier, which is a constituent of the I/V conversion means, the inversion terminal is at a ground potential due to the imaginary short-circuit. Thus, it is possible to cause the center vibration leg to be driven to vibrate even if a ground electrode is not formed as in the conventional case.